Integrated circuits can be divided into analog functions and digital functions. Most digital functions can be implemented using the following elements: combinatorial logic, clocked registers and memory. Until recently, a clear distinction was made between volatile memories such as SRAM and DRAM and non-volatile memories such as ROM, EPROM, EEPROM and Flash EEPROM. Volatile memories offer high speed and high density, but they lose data when the power supply is removed. Non-volatile memories, on the other hand, keep their data when the power supply is removed and offer relatively high density, but write times and write endurance are limited.
Today, ferroelectric memories are commercially available that provide non-volatility while retaining write performance similar to that of volatile memories. So, as ferroelectric memory technology continues to advance, the gap between volatile and non-volatile memory will continue to shrink.
Another gap exists between clocked registers and memory. Fundamentally, clocked registers, such as the multiple-purpose registers used in microcontrollers and microprocessors, are low-density memories combined with combinatorial logic in such a way as to create the desired register function. Because of the relatively late development of non-volatile memory and its poor write speed performance relative to volatile memories, clocked registers have always been volatile and most often based on cross-coupled devices similar to an SRAM cell.
Conversely, floating-gate memories require high currents to program each bit, each bit takes a long time to program, and current sensing is used to determine the state of the cell. In the current logic system design paradigm, data requiring non-volatility must either be backed up by a battery or stored in a low-write-speed non-volatile memory and restored byte-by-byte on power-up.
What is desired, therefore, is a class of logic circuits that retain the functionality and operating characteristics found in present-day integrated circuit versions of these logic circuits, while introducing the further benefit of non-volatility.